What I say stands in no contradiction with the fact that at the beginning of a walk, or at any period of a short stroll, there often comes a feeling of enhanced intellectual vigor. The parts of the brain that come into play have had no time to become tired; and besides, slight muscular exercise conduces to activity of the respiratory organs, and causes a purer and more oxydated supply of arterial blood to mount to the brain. It is most important to allow the brain the full measure of sleep which is required to restore it; for sleep is to a man's whole nature what winding up is to a clock.[1] This measure will vary directly with the development and activity of the brain; to overstep the measure is mere waste of time, because if that is done, sleep gains only so much in length as it loses in depth.[2] [Footnote 1: _Of. Welt als Wille und Vorstellung_, 4th Edition. Bk. II. pp. 236-40.] [Footnote: 2: _Cf. loc: cit_: p. 275. Sleep is a morsel of death borrowed to keep up and renew the part of life which is exhausted by the day--_le sommeil est un emprunt fait à la mort_. Or it might be said that sleep is the interest we have to pay on the capital which is called in at death; and the higher the rate of interest and the more regularly it is paid, the further the date of redemption is postponed.] It should be clearly understood that thought is nothing but the organic function of the brain; and it has to obey the same laws in regard to exertion and repose as any other organic function. The brain can be ruined by overstrain, just like the eyes. As the function of the stomach is to digest, so it is that of the brain to think. The notion of a _soul_,--as something elementary and immaterial, merely lodging in the brain and needing nothing at all for the performance of its essential function, which consists in always and unweariedly _thinking_--has undoubtedly driven many people to foolish practices, leading to a deadening of the intellectual powers; Frederick the Great, even, once tried to form the habit of doing without sleep altogether. It would be well if professors of philosophy refrained from giving currency to a notion which is attended by practical results of a pernicious character; but then this is just what professorial philosophy does, in its old-womanish endeavor to keep on good terms with the catechism. A man should accustom himself to view his intellectual capacities in no other light than that of physiological functions, and to manage them accordingly--nursing or exercising them as the case may be; remembering that every kind of physical suffering, malady or disorder, in whatever part of the body it occurs, has its effect upon the mind. The best advice that I know on this subject is given by Cabanis in his _Rapports du physique et du moral de l'homme_.[1]